1. Field of the Invention
The present invention relates to a method and a system for sorting particles of different plastic materials. More particularly, the present invention relates to a method and a system for separating and sorting plastic materials by particle size and density, as well as with selective electrostatic charges.
2. Description of Prior Art
Disposal of solid waste is an increasing problem that has reached crisis levels in some parts of the country. An obvious solution is to recycle materials that normally are landfilled. With a commercially viable separation process, a category of readily recyclable material would be waste plastics. Further, other industries, e.g. wire & cable and automotive, have a number of materials that would have value if they were reclaimable. A major obstacle to the reclamation of these materials is the presence of commingled plastic materials which are extremely difficult to separate into discrete components.
It is very difficult to cost effectively recycle commingled plastics because failure to separate the plastics completely may substantially reduce the properties of one or more of the plastic material. For example, PET and PVC materials are not compatible. PET melts at about 500.degree. F. while PVC will thermally degrade at approximately 400.degree. F. Upon degrading, PVC gives off hydrochloric acid due to dehydrochlorination which destroys desirable properties of the PET material. Specifically, discoloration, voids, and black specs may be found in the PET product as a result of the PVC which will downgrade the quality of the recycled product. Further, the evolving HCl may be harmful to personnel, and can cause corrosive wear of extrusion or other equipment.
In the plastic packaging industry, typical rigid household plastic packages, such as bottles and other containers, are primarily made of polyvinylchloride (PVC), polyethylene terephthalate (PET), polystyrene (PS), polypropylene (PP), and polyethylene (PE), including high density polyethylene (HDPE) and low density polyethylene (LDPE).
Plastics used in packaging containers are currently labeled with a recycle triangular logo formed by three arrows aligned head-to-tail and having a number within the triangle identifying the plastic in accordance with the following Table.
TABLE ______________________________________ No Plastic ______________________________________ 1 PET (PETE) 2 HDPE 3 PVC 4 LDPE 5 PP 6 PS 7 Other (co-extruded, mixed plastics, etc.) ______________________________________
Thus, plastic bottles, for example, are manually separable by the user and/or the collector. However, waste plastic packaging materials generally are commingled during collection, and the bottles and containers are either compacted or comminuted for space conservation. The primary known separation methods are water flotation or hydrocyclone processes based on density differences. These wet methods are expensive and time consuming due to costly drying. Further, water is of particular concern when the plastic is destined for extrusion pelletization.
The water flotation process generally is a float/sink operation where the lighter fractions, (e.g. PE and PP) float on the water and are culled off while the heavier fractions (e.g. PVC, PET and PS) sink. The density ranges of these materials are very close and may overlap. For example, typical PVC densities range from 1.25 to 1.36 g/cc while PET densities range from 1.32 to 1.39 g/cc.
With this overlap in densities, obtaining a satisfactory separation of PVC and PET with the water floatation method will be very difficult. Further, the water flotation process requires a relatively high amount of energy and water utilization, as well as a need to purify the water effluent. It should be noted that the use of hydrocyclones can shift the density where separation occurs so that PS can generally be recovered with the PE and PP fractions.